A compression encoding technique is used so as to efficiently transmit or accumulate moving picture data. In the case of a moving picture; the MPEG1, 2, and 4 or H.261 to H.264 systems are widely used.
In order to improve the encoding efficiency, a target picture is divided into a plurality of target blocks, and then a predictive encoding/decoding process is performed. Specifically, a picture for one frame is divided into target blocks of 16×16 pixels. In the case of an intra-frame predictive encoding method, a predicted signal is generated using an adjacent picture signal (reproduced or uncompressed from compressed picture data) which resides in the frame in which a target block to be encoded resides and is subtracted from the target block to obtain a difference signal which is then encoded.
Instead of the intra-frame predictive encoding method, an inter-frame predictive encoding method may be used (in which a predicted signal of a target block is generated, using another frame adjacent on the time axis in a moving picture, and the difference between the predicted signal and the target block is encoded). In this case, a motion detection is carried out for the target block using uncompressed encoded frames as reference pictures. A predicted signal with the minimum error is then determined, and a difference value between the determined predicted signal and the target block is obtained.
Next, discrete cosine transform and quantization processes are performed on the difference signal obtained by the intra-frame predictive encoding method or the inter-frame predictive encoding method. The quantized coefficients of discrete cosine transform and the motion vectors or mode information for specifying the predicted signal are entropy-encoded to generate encoded data.
Data encoded in the above-described manner is uncompressed and reproduced so as to obtain a predicted signal of a next target block. The reproduced picture is stored as a reference picture in a frame memory after distortion caused by encoding is removed by means of in-loop filter processing.
The in-loop filter may be a known block distortion removing filter which removes a block distortion between blocks. However, the block distortion removing filter has a problem in that the filter is prone to generate an out-of-focus picture. Meanwhile, below-described Patent Literature 1 discloses a filtering process which removes noise near edges in a reproduced picture. According to the method of Patent Literature 1, since the filtering process is performed so as not to cross over the edges of an object within a picture, it is effective to clear the impressions of the entire picture while preserving the edges.